It has become desirable to employ non-impact printers for text and graphics. Xerographic techniques are employed in such non-impact printers. An electrostatic charge is developed on the surface of a moving drum or belt and selected areas of the surface are discharged by exposure to light. Alternatively, areas may be charged by illumination. A printing toner is applied to the drum and adheres to the areas having an electrostatic charge and does not adhere to the discharged areas. The toner is then transferred to a sheet of plain paper and is heat-fused to the paper. By controlling the areas illuminated and the areas not illuminated, characters, lines and other images may be produced on the paper.
One type of non-impact printer employs an array of light emitting diodes (LEDs) for exposing the photoreceptor drum surface. A line of minute LEDs is positioned next to a lens so that the images of the LEDs are arrayed across the surface to be illuminated. In some printers, multiple rows of LEDs may be used. As the surface moves past the line of LEDs, the LEDs are selectively activated to either emit light or not, thereby exposing or not exposing the surface of the drum in a pattern corresponding to the LEDs activated.
To obtain good resolution and image quality in such a printer, the physical dimensions of the LEDs must be quite small and very tight position tolerances must be maintained. Dimensional tolerances are often no more than a few micrometers.
At the lowest level of integration, a plurality of light emitting diodes are formed on gallium arsenide chips or dice by conventional techniques. The size and positions of the LEDs are controlled by well-established photolithographic techniques. The wafer on which the LEDs are formed is carefully cut into individual dice, each having a row of LEDs. In an exemplary embodiment, the length of such a die is cut to .+-.2 micrometers and the width is cut to .+-.5 micrometers. An exemplary die about 8 millimeters long may have 96 LEDs along its length.
Practical problems arise in arranging these LED-bearing dice in a line with the necessary precision for good image quality. Clearly economical as well as precise assembly techniques are important.
For purposes of exposition herein, the face of the LED die on which the LEDs are formed is referred to as the front and the opposite face as the back. The same nomenclature is used for the other parts of the assembly such as integrated circuit chips, mounting tiles and the like. In each case, the face facing in the same direction as the LEDs is referred to as the front.
It is also convenient to employ a coordinate system for the assembly. Thus, the x direction is along the line of LEDs. The y direction is in the plane of the LEDs perpendicular to the x direction. The z direction is normal to these and is the direction in which the light output from the LEDs is generally directed. It might be thought of as the height.
In an exemplary embodiment, a print-head with a length corresponding to the width of a sheet of business size paper has 2592 light emitting diodes. Close control of dimensions between adjacent LEDs is more significant than the total length of the array since the user is more sensitive to a line displacement or character imperfection in mid-page than a discrepancy in the total page width. Spacing of LEDs on a die is well controlled by photolithography. The spacing between LEDs at the ends of adjacent dice is an area of concern in assembling an LED print head. Typical tolerance between adjacent LEDs at the ends of dice can be as little as .+-.15 micrometers in the x direction.
Similarly, the tolerance in the y direction may be .+-.25 micrometers at the ends of adjacent dice, with a total "waviness" along the entire print-head of .+-.75 micrometers. Tolerance in the z direction may be .+-.25 micrometers to assure that light from the LEDs is sharply focused on the photoreceptor surface throughout the full length of the array.
A significant problem may be encountered in the assembly of print heads due to close tolerances in the x direction. One qualification test for print heads involves temperature cycling between -30.degree. C. and 65.degree. C. In an exemplary embodiment the LED dice are basically gallium arsenide. A row of LED dice are mounted on a stainless steel tile. A row of such tiles are assembled on an aluminum substrate referred to as a mother plate. Gallium arsenide has a coefficient of thermal expansion as low as 3.8.times.10.sup.-6 /.degree. C. The coefficient of thermal expansion of a representative aluminum alloy is 23.6.times.10.sup.-6 /.degree. C. The coefficient of thermal expansion of the steel tiles is in between these extremes.
When such a print head assembly is subjected to thermal cycling to low temperature, an LED die at the edge of one tile may "crash" into the LED die at the edge of the adjacent tile. Pressure between adjacent dice may cause chipping or cracking of such a die, which may damage one or more LEDs or their electrical connections. As many as 10% of print heads may show such cracking or chipping due to the adjacent LEDs being too close together. On the other hand, the dice cannot be spaced too far apart since broad spacing may leave a noticeable gap. Thus, there is a very tight tolerance on spacing of dice on the print head. An appreciable number of print heads fail to meet the upper limit of specified tolerance.
It is desirable to minimize the problem of contact between LED dice on assembled print heads and relax the stringency of the spacing tolerances. However, any solution to this problem should not, itself, have an adverse effect on cost or reliability. Some increase in cost is, of course, tolerable if reliability is sufficiently enhanced. It is important that the x, y and z tolerances are not compromised. Furthermore, a solution to this problem should not introduce different problems for other reliability testing such as high temperature soaking, vibration tests and the like.